The present invention relates to a method of calibrating a network analyzer.
The measurement accuracy of network analyzers may be considerably improved by system error correction. Such system error corrections are obtained by calibration measurements which are realized in such a way that instead of the device under test (DUT) several calibration standards are successively connected between the two externally accessible test ports of the network analyzer where the transmission and reflection parameters will then be measured. The calibration measurement procedures used so far are based on a simplified two-error two-port model as disclosed in U.S. Pat. No. 4,982,164 which is hereby incorporated by reference.
FIG. 1 is the schematic circuit diagram of a commonly used network analyzer (for instance the network analyzer ZPV-Z5 of Rohde & Schwarz) in which a three-port 2, for example, a change-over switch, feeds two separate test circuits 12 and 13 from a radio frequency generator 1 capable of being tuned through a predetermined frequency range. The two test circuits 12 and 13 which are alternatingly turned on lead to four-ports 4 and 5 which are configured as bridges or directional couplers and which have signal detectors 8, 9 and 10, 11 respectively connected thereto by means of which voltage measurements as to magnitude and phase can be performed. These signal detectors may be mismatched. The four-ports 4 and 5 also have test ports 6 and 7 connected thereto between which a two-port 3 may be connected as the device under test (DUT). Hence, by way of the signal detectors 8, 9 and 10, 11 it is possible to measure the complex reflection factors S.sub.11 and S.sub.22 as well as the complex transmission factors S.sub.12 and S.sub.21 in forward and backward direction at the input and the output of an interconnected DUT 3. The four measured complex scattering parameters S.sub.11, S.sub.22, S.sub.12 and S.sub.21 fully describe a linear two-port for any frequency, and any further unknown quantities of interest may be determined from these values.
In accordance with the so-called two-error two-port model, several calibration standards instead of the DUT 3 are successively connected between the test ports 6 and 7 for calibrating purposes. In this way the scattering parameters are again obtained from which the correction values are calculated, which are stored in a memory in the network analyzer and used correspondingly for subsequent device testing.
However, this known error model for the usual two-port calibration is incomplete because it does not take into account any possible coupling between the components of a DUT to be calibrated. When so-called on-wafer networks (active or passive networks configured on semiconductor substrates) are calibrated it is necessary, for instance, to also include in the calibration measurement the coupling of the circuit portions existing between the test ports of the network analyzer and the actual DUT (for instance a transistor of the semiconductor circuit). This general calibration problem for the total error model is schematically illustrated in FIG. 2, the coupling between the components of the DUT and the externally accessible test ports 6 and 7 of the network analyzer, which coupling must not be neglected, is illustrated as an undefined free space which, as shown in FIG. 3, may be illustrated in terms of an error network C interconnected between the DUT 3 and the test ports 6 and 7.
The theory for solving the specified calibration problem is known (SPECIALE, R.A., A Generalization of the TSD Network analyzer Calibration Procedure, Covering n-Port Scattering-Parameter Measurements, Affected by Leakage Errors, IEEE Transactions on Microwave Theory and Techniques, MTT-25, December 1977, pp. 1100-1115). Based on the above theory it has already been proposed to solve the calibration problem with four successive calibration measurements (HEWLETT PACKARD, 16-Term Error Model and Calibration Procedure for On-Wafer Network Analysis Measurements, IEEE Transactions on Microwave Theory and Techniques, MTT-39, December 1991, pp. 2211-2217). The first calibration standard used is a direct or through-connection of the two test ports (through), the second calibration measurement is the concurrent connection of a reflection-free termination impedance to both test ports (so-called double-one-port calibration measurement Match-Match), the third calibration measurement comprised an open on one test port and a short on the other test port (double-one-port measurement Open-Short), and the fourth calibration measurement finally was the reverse sequence, i.e. a short on the first test port and an open on the second test port (double-one-port measurement Short-Open). This known 16-term calibration procedure, as it is called, will not yet completely solve the mentioned calibration problem since the successive four calibration measurements permit only 16 equations to be set up for error computation, and some of these equations are dependent on each other so that there are at most 14 independent equations whereby the problem cannot be solved.